A variety of particle and droplet characterizing techniques involve the analysis of fringe patterns or oscillation patterns scattered by an irradiated particle or droplet. For example, interferometric particle sizing is based on the interference of laser energy reflected and refracted from transparent spherical particles. The angular spacing of the interference fringes is inversely proportional to the droplet diameter.
Some interferometric particle sizing systems utilize a sheet-shaped laser beam that is applied to a measurement space, and out-of-focus images of droplets irradiated with the laser beam are captured. A known method for fringe pattern analysis in the out-of-focus images involves the so-called “edge detection” technique. The out-of-focus image corresponding to each droplet is distinguished and extracted from the original image by detecting the edge of the fringe pattern. Since the relationship between the angular spacing of the fringe pattern and the diameter of the droplet is known, the diameter of the droplet is determined by counting the number of interference fringes in the extracted fringe pattern.
U.S. Pat. No. 6,587,208 to Maeda, et al. (Maeda) discloses an improvement on the edge detection technique. The Maeda method calls for the use of a cylindrical lens to compress an otherwise disk-shaped fringe pattern into a line oscillation pattern without significantly altering the spacing of the fringes. The line oscillation pattern enables the use of a low-pass filter to convert the oscillation pattern to a one-dimensional Gaussian pattern, from which individual fringe patterns can be distinguished and extracted. Maeda also discloses the use of a fast Fourier transform to establish the number of fringes in an extracted fringe pattern.
Another technique that may involve fringe pattern analysis is laser Doppler velocimetry (LDV). Particles pass through a viewing volume determined by the intersection of two laser beams. Light scattered by the particles passing through the viewing volume is detected, and the particle velocity, which is inversely proportional to the frequency of the scattered fringe pattern, is determined.
A problem arises with the aforementioned sizing and velocity techniques when the population density of a particle or droplet field is high enough to cause frequent overlapping of corresponding fringe patterns. Existing techniques cannot discern between overlapping fringe patterns, and instead presumes the overlapping fringe patterns to be a single fringe pattern. An error results in the fringe count (and therefore the size determination) as well as in the frequency and position determination of the corresponding particles.